The present invention relates generally to a charged particle beam apparatus and specifically to techniques for automatically adjusting for astigmatism.
An electron microscope is used as an automatic inspection system for inspecting and measuring a fine circuit pattern created on a substrate such as a semiconductor wafer. In the case of a defect inspection, an electron beam image detected by a scanning electron microscope is acquired and compared with a reference image used as a comparison standard. In a measurement of a hole diameter or a fine circuit pattern used in monitoring and setting a manufacturing process condition of a semiconductor device, measurement of a length is based on image processing of an electron beam image detected from the scanning electron beam microscope. In an inspection to detect a defect of a pattern by comparison of an electron beam image of the pattern with a reference image and in a measurement of a line width of a pattern by processing an electron beam image, the quality of the electron beam image greatly affects the reliability of the result of the inspection and the measurement.
The quality of an electron beam image deteriorates due to causes such as astigmatism of the electron beam system and degradation of the resolution attributed to defocusing. A poor quality electron beam image causes the inspection sensitivity and the performance of the length measurement to deteriorate. In addition, in such an image, the pattern width varies and a result of detection of an image edge can not be obtained in a relatively stable manner. In consequence, results of measuring the wire width of a pattern and the diameter of a hole with such a poor quality beam will often be unsatisfactory.
Conventionally, the focal point and the astigmatism of an electron beam optical system are adjusted by properly changing a control current of an objective lens and control currents of 2 coil sets each comprising a plurality of astigmatism correction coils while visually observing an electron beam image. To be more specific, the focal point is adjusted by varying the convergence height of a beam. The convergence height of a beam is changed by adjusting a current flowing through the objective lens.
While there are perceived advantages, it can take time to execute the conventional technique of adjusting a control current of an objective lens and control currents of 2 coil sets each comprising a plurality of astigmatism correction coils while visually observing an electron beam image as described above. In addition, the conventional techniques often require that the surface of a sample be scanned by using an electron beam several times. As a result, it is quite within the bounds of possibility that a problem of a damage inflicted on the sample arises. In addition, since in conventional systems, adjustments are often carried out manually, the result of the adjustment varies from operator to operator. Moreover, the astigmatism and the focal position can change with time. It is thus necessary to adjust the astigmatism and the focal position periodically by manual operations in order to carry out an automatic inspection and an automatic measurement of a length.
What is needed are automated techniques for controlling electron beams.